Hello,
@spip
The issue you described is Plesk bug PPPM-15074, that fixed in Plesk version 18.0.73
https://docs.plesk.com/release-notes/obsidian/change-log/
Fixed the issue where the amount of traffic consumed by domains was not being updated on the “Domains” page. (PPPM-15074)
To resolve the issue try to update Plesk, refer this article for setails
How to upgrade Plesk to the next release
Please note that traffic and disk space usage is updating during daily maintenance task. So after Plesk be upgraded to 18.0.73 you should wait for daily maintenance task execution.
To force usage updating after the Plesk upgrade , connect via SSH as root and execute
# plesk daily -f ExecuteStatistics